Re: [PATCH v3] Btrfs: add skeleton code for compression heuristic

[Anand Jain <anand.jain@oracle.com>]

On 28/07/2017 00:36, David Sterba wrote:
> On Mon, Jul 24, 2017 at 11:40:17PM +0800, Anand Jain wrote:
>>
>>> Eg. files that are already compressed would increase the cpu consumption
>>> with compress-force, while they'd be hopefully detected as
>>> incompressible with 'compress' and clever heuristics. So the NOCOMPRESS
>>> bit would better reflect the status of the file.

  I thought 'compress' in above, is the compress option. Ah you mean
  to say compression algo .. got it. Right compress-force for
  incompressible-data is very expensive.

  And its also true that compress option for incompressible data is
  not at all expensive and its only one time.

>>    current NOCOMPRESS is based on trial and error method and is more
>>    accurate than heuristic also loss of cpu power is only one time ?


> Curreently, force-compress beats everything, so even a file with
> NOCOMPRESS will be compressed, all new writes will be passed to the
> compression and stored uncompressed eventually.

  It makes sense to me when you replace NOCOMPRESS with
  incompressible-data in the above statement. As in my understanding..

  You will never have a file with NOCOMPRESS flag if compress-force
  option is used.


> Each time they
> compression code will run and fail, so it's not one time.
>
> Although you can say it's more 'accurate', it's also more expensive.

   yes. Expensive only in compress-force.

>>    May be the only opportunity that heuristic can facilitate is at the
>>    logic to monitor and reset the NOCOMPRESS, as of now there is no
>>    such a logic.
>
> The heurictic can be made adaptive, and examine data even for NOCOMPRESS
> files, but that's a few steps ahead of where we are now.

   Nice.

Thanks, Anand